As is well known, prospecting for minerals of commercial or other value (including but not limited to hydrocarbons in liquid or gaseous form; water e.g. in aquifers; and various solids used e.g. as fuels, ores or in manufacturing) is economically an extremely important activity. For various reasons those wishing to extract such minerals from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the minerals in a geological formation and also any difficulties that may arise in the extraction of the minerals to surface locations at which they may be used.
For this reason over many decades techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before mineral extraction activities commence and also, increasingly frequently, while they are taking place.
Broadly stated, one form of logging involves inserting a logging tool (also sometimes called a “sonde”) into a borehole or other feature penetrating a formation under investigation; and in most sonde designs using the sonde to energise the material of the rock, etc., surrounding the borehole in some way. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
In another form of logging, known as logging-while-drilling (LWD), energising and detecting elements are supported in a collar either as a recognisable sonde or by reason of being supported in some other way such as a distributed arrangement.
Notwithstanding the constructional differences of LWD logging equipment compared with more conventional logging sondes, in the LWD equipment a process of energising rock surrounding a borehole and detecting returned energy is also practised.
The passage of the energy through the rock alters its character. Knowledge of the attributes of the emitted energy and that detected after passage through the rock may reveal considerable information about the chemistry, concentration, quantity and a host of other characteristics of minerals and associated fluids in the vicinity of the borehole, as well as geological aspects that influence the ease with which the target mineral material may be extracted to a surface location.
Logging techniques are employed throughout the mining industries, and also in particular in the oil and gas industries. In the logging of oil, coal and gas fields (including fields combined with rock types such as shales) specific problems can arise. Broadly stated, this is because it is helpful to consider a geological formation that typically is porous and that contains a hydrocarbon-containing fluid such as oil or gas or (commonly) a mixture of fluids perhaps only one component of which is of commercial value.
This leads to various complications associated with determining geological attributes of the oil or gas field in question. In consequence a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that as outlined above is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling; or a collar also as outlined above.
Typically, as noted, the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question).
The borehole usually is several hundreds or thousands of feet in length yet is narrow (being perhaps as narrow as 3 inches (about 76 mm) or less in diameter), although in practice such a borehole is almost never of uniform diameter along its length. A borehole normally is formed by drilling using a drill bit fed into the borehole on drillpipe, although other techniques occasionally are used.
One general class of logging is intended to produce so-called image logs, which are graphical representations of the rock in the vicinity of a borehole. Image logs are not optical images, and instead are generated by sending typically an electrical current, an acoustic signal or nuclear radiation from a sonde or other emitter into the rock; and processing the returned energy as electrical or electronic signals in a way that produces coloured plots in which different regions represent different geological features.
Of particular interest to geologists who study image logs is the identification of layer-like features. These can represent fractures, beds, the edges of beds and similar phenomena that either may help to indicate the likely presence of materials of value; or may indicate potential problems in the extraction of such materials.
The creation of an image log involves the operation of a sonde or other emitter, as indicated, while it is being withdrawn along a borehole towards a surface location or (in some cases) conveyed from a surface location into the borehole; or while the borehole is being drilled. The sonde energises the rock in an azimuthal pattern as it moves, with the result that the rock surrounding the borehole is logged at spaced intervals along a chosen length of the borehole and at points around the circumference of the generally circular cross-section of the borehole.
This results in a two-dimensional set of log data values that are processable as electrical or electronic signals. The co-ordinates of the individual values can be presented as borehole depth (i.e. distance along the borehole) and azimuthal (i.e. angular distance around the borehole) co-ordinates. Such a co-ordinate system is referred to herein as a cylindrical co-ordinate system, and references to a cylindrical frame of reference may be construed accordingly. In this regard the concept of the “frame of reference” of a co-ordinate system is extremely familiar to geologists and geoscientists, and is made use of routinely by such workers.
When the signals representative of the data are processed to produce a coloured image as mentioned above this too necessarily is two-dimensional, and resembles a hollow cylinder that has been cut lengthwise and “unrolled” to present a flat image.
An edge, bed or layer feature that intersects the borehole at anything other than a right angle appears as a sinusoid or, more typically, (as a result of inconsistencies in the borehole and in the logging process) a distorted, partially occluded or otherwise incomplete sinusoid the line thickness of which may not be constant in such a flat image plane. Such imperfect sinusoids are hard to interpret correctly and yet are commonplace. A human viewer assessing the two-dimensional image must seek to envisage the layer feature in the three-dimensional space that gave rise to the sinusoid in the two-dimensional rendition. This is difficult to achieve reliably or in a reasonable timescale.
Most machine viewers are also likely to be inefficient at identifying the sinusoids.
This is partly because there may be many edge, bed, or layer features, caused by differing phenomena, in the length of borehole logged. This results in a confusing superimposition of one sinusoid on another in the two-dimensional image log. Neither machine viewers nor humans are very good at discriminating between different features in such conditions.
Some of the sinusoids are geologically more significant than others, yet hitherto machine-based viewing methods have been poor at discriminating between important and insignificant features.
In addition to the foregoing the existing image logging techniques produce an image of the rock at the interface defining the outer extremity of the borehole. Prior art image logging methods are not capable of interpolating to produce data on the approximately cylindrical region of rock that is removed during the process of drilling or otherwise forming the borehole. Moreover the prior art has not provided any good way of synthesizing images of cores, i.e. discrete cylindrical or essentially cylindrical sections of rock used e.g. for assessing various qualities of the rock in which a borehole is to be drilled or has been drilled.
More generally it is desired to produce image logs that more realistically represent the three-dimensional reality “down hole” in a borehole than the existing two-dimensional renditions that are currently available.
What is needed is a way to overcome or at least ameliorate one or more drawbacks of the prior art.